In modern optical communications networks, it is generally desirable to transmit optical signals at high power levels in order to maintain sufficient signal to noise ratios over extended transmission distances, and thereby obtain an acceptably low Bit Error Rate (BER) in a received optical signal.
However, conventional optical waveguides (such as optical fibers) comprise an optical transmission medium which exhibits nonlinear effects at high optical power levels, resulting in degradation of the optical signal. Nonlinear effects may similarly occur within optical terminals of the system, in optical transmission media or in components such as optical amplifiers. The optimum power level at which optical signals can be transmitted is typically the maximum power level at which significant degradation due to nonlinearity is avoided. Since the performance of various optical components within the system varies with operating conditions, age, and component replacement, a safety margin is used in setting the maximum power level. Consequently, optical communications systems typically operate at power levels which are less than the optimum power level. A detailed discussion of nonlinear optical effects is provided by Agrawal, Govind P., “Nonlinear Fiber Optics”, 2nd. Ed., Academic Press, Inc., San Diego, Calif., 1995 (ISBN 0-12-045142-5).
Of particular concern in considering nonlinear processes are the effects of phase nonlinearities, which increase as data rates and optical power levels increase, and which ultimately limit both system performance and signal reach.
Phase nonlinearities are the result of complex interactions between the optical power present in the fiber; the refractive index of the fiber medium, including the non-linear index coefficient; the wavelength division multiplexing (WDM) channel spacing; the polarization states of the signals within each of the channels; and the proximity of channel wavelengths to the zero-dispersion wavelength of the fiber. Phase nonlinearities include self-phase modulation (SPM), cross-phase modulation (XPM), and modulation-instability (MI), all of which are discussed in detail in Agrawal (supra), at chapters 4 and 7.
Self-phase modulation (SPM) is a by-product of the relationship between the refractive index of the fiber medium and the optical power present in the fiber. In particular, changing optical power causes a change in the refractive index of the fiber medium. The refractive index change is proportional to the optical power level. Changing the refractive index produces a Doppler-like frequency shift (or chirp) that is proportional to the time-rate of change of the refractive index (and, equivalently, the optical power level). Thus, changing optical power levels due to modulation of an optical signal causes a frequency-shift (or chirp) within the signal itself. For example, consider an isolated signal pulse (e.g., an isolated binary “1”) launched into the optical fiber. SPM results in the leading edge of the pulse being red-shifted (that is, frequency shifted toward the red end of the optical spectrum), and the trailing edge of the pulse blue-shifted. Chromatic dispersion of the fiber will then cause these red- and blue-shifted portions of the pulse to propagate through the fiber at different speeds, which may result in time-domain distortion of the original pulse shape.
As may be appreciated, because the magnitude of the frequency shift is proportional to the time-rate of change of the optical power level, the amount of red- and blue-shift experienced by the pulse edges will be a function of the rise and fall times at the leading and trailing edges, and the peak power level of the pulse. In additional to these factors, the total time-domain distortion experienced by the pulse will also be affected by the nominal length of the pulse, and the length of the fiber before signal detection and regeneration. Clearly, the effects of SPM become increasingly severe as signal power, data rate (or spectral efficiency), and fiber span length are increased.
Cross-phase modulation (XPM) is similar to SPM, and produces the same frequency-shifting effects, but occurs in Wavelength Division Multiplexed (WDM) systems. XPM is always accompanied by SPM, and occurs because the effective refractive index “seen” by an optical wave propagating in the fiber medium depends not only on the intensity of that wave but also on the intensity of other co-propagating waves. Thus, refractive index changes due to rising and falling optical power levels in one channel induce corresponding frequency-domain distortions (chirps) within co-propagating signals (in adjacent channels). Chromatic dispersion of the fiber may then induce time-domain distortions of those signals, in the same manner as described above.
Modulation instability (MI) is an XPM-induced interaction between co-propagating optical waves (whether due to signal traffic, noise, or pump laser signals) within the optical fiber. This interaction produces new, unwanted wavelengths (or side-bands) that can interfere with, and/or couple power from, desired optical signals.
Nonlinear effects in an optical fiber can be measured using known optical signal and spectrum analysis equipment. Respective channels of a Wavelength Division Multiplexed (WDM) communications system can be monitored, either by multiple signal analyzers arranged in parallel, or using a single signal analyzer that is sequentially tuned to receive each optical channel signal in turn. Optical Spectrum Analyzers (OSAs) can be used to determine average and peak power levels, as a function of wavelength, across a desired range of wavelengths. Known analytical techniques can be used to determine non-linear effects from the data measured by these systems.
Due to their cost and complexity, conventional optical signal and spectrum analysis equipment is typically restricted to laboratory use. Furthermore, accurate measurement of nonlinear effects using such equipment typically requires specialized test set-ups, which, again, can only be provided in a laboratory setting.
In order to monitor nonlinearities in installed optical communications systems, simpler and less expensive monitoring equipment is required. Typical (in situ) optical performance monitoring systems known in the art are disclosed in co-assigned U.S. Pat. Nos. 5,513,024; 5,949,560; 5,999,258; 6,128,111; 6,222,652; and 6,252,692. While these systems enable some degree of performance monitoring, they tend to suffer a number of disadvantages. In particular, per-channel monitoring systems are typically dependent on a low frequency pilot tone (or dither) having known parameters. Any error between the design and actual parameter values of the launched pilot tone will naturally degrade the accuracy of any performance parameters calculated at the monitoring point. Additionally, this approach assumes that performance parameters calculated on the basis of the low frequency pilot tone will be valid for the high-speed data traffic. Consequently, frequency-dependent effects (most notably phase nonlinearities) cannot be detected with this arrangement. Finally, the detectors and signal processors utilized in these monitoring systems are low frequency analog devices. This precludes their use for monitoring high-frequency phenomena such as SPM, XPM and MI.
Accordingly, a method and system that enables efficient monitoring of phase nonlinearities in an installed optical communications system remains highly desirable.